Differential subcellular localization of human MutY homolog (hMYH) and thefunctional activity of adenine : 8-oxoguanine DNA glycosylase

The post-replicative adenine:8-oxo-7,8-dihydroguanine (GO) mismatch is cruc ial for G:C to T:A transversion. This mismatch is corrected by Escherichia coil MutY which excises the adenine from A:GO. A candidate gene coding for the human counterpart of MutY has been cloned as hMYH. However, the functio n and enzyme activities of the gene product have not been identified. We pr eviously demonstrated that an epitope-tagged hMYH protein behaves as a mito chondrial protein. In the present study, we have identified an alternative hMYH transcript, termed type 2, which differs in the exon 1 sequence of the known transcript (type 1). A nuclear localization for the type 2 protein w as revealed by detection of epitope-tagged protein in COS-7 cells. Expressi on of both type 1 and type 2 transcripts was reduced in postmitotic tissues . hMYH cDNA suppressed the mutator phenotype of E.coli mutY. In vitro expre ssed hMYH showed adenine DNA glycosylase activity toward the A:GO substrate . The protein can bind to A:GO, and to T:GO and G:GO without apparent catal ysis. These results represent the first demonstration of the function of th e hMYH gene product which is differentially transported into the nucleus or the mitochondria by alternative splicing.